<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
 <style>
  body, html {
   width: 100%;
   height: 100%;
   margin: 0;
  }
 
  #container {
   width: 200px;
   height: 200px;
   margin-left: 100px;
   padding: 15px;
   border: #00cdcd 2px solid;
   box-sizing: border-box;
  }
 
  .item {
   cursor: default;
   width: 100%;
   height: 100%;
   background: #757575;
  }
 </style>
</head>
<body id="body">
<div id="container">
 <div class="item"></div>
</div>
<script>
 //需要调整尺寸的div
 let c = document.getElementById('container')
 // body监听移动事件
 document.getElementById('body').addEventListener('mousemove', move)
 // 鼠标按下事件
 c.addEventListener('mousedown', down)
 // 鼠标松开事件
 document.getElementById('body').addEventListener('mouseup', up)
 
 // 是否开启尺寸修改
 let resizeable = false
 // 鼠标按下时的坐标，并在修改尺寸时保存上一个鼠标的位置
 let clientX, clientY
 // div可修改的最小宽高
 let minW = 8, minH = 8
 // 鼠标按下时的位置，使用n、s、w、e表示
 let direc = ''
 
 // 鼠标松开时结束尺寸修改
 function up() {
  resizeable = false
 }
 
 // 鼠标按下时开启尺寸修改
 function down(e) {
  let d = getDirection(e)
  // 当位置为四个边和四个角时才开启尺寸修改
  if (d !== '') {
   resizeable = true
   direc = d
   clientX = e.clientX
   clientY = e.clientY
  }
 }
 
 // 鼠标移动事件
 function move(e) {
  let d = getDirection(e)
  let cursor
  if (d === '') cursor = 'default';
  else cursor = d + '-resize';
  // 修改鼠标显示效果
  c.style.cursor = cursor;
  // 当开启尺寸修改时，鼠标移动会修改div尺寸
  if (resizeable) {
   // 鼠标按下的位置在右边，修改宽度
   if (direc.indexOf('e') !== -1) {
    c.style.width = Math.max(minW, c.offsetWidth + (e.clientX - clientX)) + 'px'
    clientX = e.clientX
   }

   // 鼠标按下的位置在上部，修改高度
   if (direc.indexOf('n') !== -1) {
    c.style.height = Math.max(minH, c.offsetHeight + (clientY - e.clientY)) + 'px'
    clientY = e.clientY
   }
   // 鼠标按下的位置在底部，修改高度
   if (direc.indexOf('s') !== -1) {
    c.style.height = Math.max(minH, c.offsetHeight + (e.clientY - clientY)) + 'px'
    clientY = e.clientY
   }
 
   // 鼠标按下的位置在左边，修改宽度
   if (direc.indexOf('w') !== -1) {
    c.style.width = Math.max(minW, c.offsetWidth + (clientX - e.clientX)) + 'px'
    clientX = e.clientX
   }
 
  }
 }
 
 // 获取鼠标所在div的位置
 function getDirection(ev) {
  let xP, yP, offset, dir;
  dir = '';
 
  xP = ev.offsetX;
  yP = ev.offsetY;
  offset = 10;
  if (yP < offset) dir += 'n';
  else if (yP > c.offsetHeight - offset) dir += 's';
  if (xP < offset) dir += 'w';
  else if (xP > c.offsetWidth - offset) dir += 'e';
 console.log(xP,c.offsetHeight - offset);
  return dir;
 }
</script>
</body>
</html>